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Monday, June 11, 2007

With the spring migration settling down and the breeding season underway, we still have a few weeks of bird song left. It seems as good a time as any to review some books on bird song. A few weeks ago I posted some ways to learn identification of birds by their songs and calls. Today I would like to introduce a book about the functions of bird song, The Singing Life of Birds by Donald Kroodsma. Many readers may have encountered this book previously when it was first published two years ago. The book is now in paperback, which is the edition that I read.

The book includes a compact disk with examples of every song referenced in the text. Several tracks give slower versions to make certain aspects of a song more obvious, such as the details of a fast song like a winter wren's or the harmonics in a wood thrush's song. Printed sonagrams illustrate most tracks on the CD. The sonagrams make it possible to follow the details of a bird's song even without listening to the recordings. (Of course, the text is more meaningful if you listen as well as read the sonagrams.) The book's introductory material discusses how to read sonagrams and how they are used in research.

This book does not teach identification, but the development and function of songs over the course of birds' life histories. The Singing Life of Birds is divided into chapters on how bird songs develop, extremes of bird song, dawn songs, and female songs. Individual chapters draw heavily from Kroodsma's own research, as one can confirm by perusal of the book's notes and bibliography. At times the book seems to be as much about Kroodsma's methods as about bird songs. Kroodsma describes the equipment he uses for research (parabolic reflector and audio recorder) and how he analyzes the recordings visually through the use of sonagrams.

Some birds learn their songs from other adult birds, and some birds inherit them genetically. How birds develop their songs is revealed through study of song variety and the development of local dialects. Learned songs tend to be learned from future competitors rather than from parents. Young male birds disperse within weeks of leaving the nest and then spend the late summer and autumn months seeking a territory of their own. During that process, they learn the songs of the males around their prospective territories.

Learning from future competitors rather than parents offers some advantages to the young birds. Matched countersinging, which requires neighboring males to know the same songs, may serve as a way to affirm territorial boundaries or intimidate other males. It is also possible that females choose a male with the greatest variety of songs and with the best versions of the matched songs.

Certain birds do not learn songs. Among them are most of the suboscine passerines, represented in the northern hemisphere by tyrant flycatchers. Birds that do not learn their songs have no local dialects because their songs are genetically imprinted. This is useful knowledge for birders since we can be more confident in our identifications of Empidonax flycatchers if we know that a willow flycatcher will never sing the same song as an alder flycatcher.

Moving from the complex questions of learning and development, The Singing Life of Birds veers into appreciation. Kroodsma asks why some songs are so complex and why others sound so beautiful to our ears. The questions are not always answered completely, but he analyzes each song to bring out interesting details. Among the most complex songs are those of mockingbirds and thrashers, whose repertoires may include hundreds of imitated songs, and the winter wren, one of my favorites. Wood and hermit thrushes, two of the species picked to represent especially beautiful songs, appear to have "rules" for the ordering of their songs, including how often one should be repeated. Thrush songs are also known for their internal harmonies, which Kroodsma explores in this chapter.

In most bird species, only the male sings. Females will also vocalize, but only with contact calls and chip notes. In a very few species, females will sing to communicate with their mates. Kroodsma offers the examples of Carolina wrens, northern cardinals, and barred owls. In the section on barred owls, Kroodsma introduces an alternate method for separating male and female voices. Females are not only higher pitched, but also use more vibrato on the "you-all" notes. I tried to listen for this during the bioblitz owling trip, but the differences turned out to be too subtle for me to distinguish in one night's listening.

The Singing Life of Birds is easily the best book I have reviewed so far for this blog. (Clare was similarly enthusiastic.) Kroodsma is an effective narrator, and effectively mixes tales of field research with expert insights into the complex world of bird song. The book is so rich in detail that it is hard to absorb all of its insights in one reading. I may return to some of its chapters in the future to review details that I missed the first time through. The beauty of The Singing Life of Birds is that it makes me hungry to listen more carefully to birds' songs and to learn more about how and why they sing.